Conventionally, an in-vehicle radar device which irradiates both a predetermined area in front of a vehicle and a predetermined area on a road surface is known (see Patent Document 1).
This in-vehicle radar device irradiates both the predetermined area in front of a vehicle and the predetermined area on a road surface by causing each of plural reflective surfaces of a rotating polygonal mirror to sequentially reflect a laser beam emitted from a single laser diode.
The plural reflective surfaces of the polygonal mirror includes five planar reflective surfaces used to irradiate the predetermined area in front of a vehicle in a traveling direction of the vehicle, and a concave reflective surface used to irradiate the predetermined area on a road surface.
When the in-vehicle radar device irradiates the predetermined area on a road surface, the in-vehicle radar device causes the concave reflective surface of the polygonal mirror to reflect downward and converge the light emitted from the laser diode so that an area on the road surface irradiated by the laser beam irradiation contracts.
In this way, while ensuring that the in-vehicle radar device irradiates a white line on a road surface with the laser beam, the in-vehicle radar device narrows down a beam divergence angle, contracts an irradiated area, and increases a power density within the irradiated area. As a result, the in-vehicle radar device improves a detection sensitivity of the white line on the road surface.